Room-Temperature High-Frequency Transport of Dirac Fermions in Epitaxially Grown ${\mathrm{Sb}}_2{\mathrm{Te}}_3$- and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$-Based Topological Insulators

We report on the observation of photogalvanic effects in epitaxially grown ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ three-dimensional (3D) topological insulators (TI). We show that asymmetric scattering of Dirac fermions driven back and forth by the terahertz electric field results in a dc electric current. Because of the “symmetry filtration” the dc current is generated by the surface electrons only and provides an optoelectronic access to probe the electron transport in TI, surface domains orientation, and details of electron scattering in 3D TI even at room temperature.

Figure 1

(a) Sample sketch. (b) AFM surface scan of ${\mathrm{Sb}}_2{\mathrm{Te}}_3$. (c) and (e) ARPES measurement of $p$-type ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ and $n$-type ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. (d) Structure of $(\mathrm{Sb},\mathrm{Bi}{)}_2{\mathrm{Te}}_3$ layer.

Figure 2

Model of the PGE excited in surface states of $(\mathrm{Sb},\mathrm{Bi}{)}_2{\mathrm{Te}}_3$ due to asymmetry of elastic scattering by wedges.

Figure 3

Normalized photocurrents $J_x/I$ and $J_y/I$ excited with $f=2.03\mathrm{THz}$ for front (left panels) and back (right panels) illumination of the ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ sample (a),(c) and the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ sample (b),(d) at room temperature. Solid lines show fits after Eqs. (3) with phase shifts $3\mathrm{\Phi }=-12°$ and 90°, for (a),(c) and (b),(d), respectively. Insets sketch the setups.

Figure 4

(a) Two possible domain orientations in ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ illustrated by solid and dashed violet lines connecting top Sb atoms [7]. (b) X-ray diffraction data obtained for the ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ sample showing that one domain orientation (highlighted by solid violet lines) dominates. It also reveals that the axis $x_0$ in this sample is tilted by the angle $\mathrm{\Phi }=-4°$ in respect to the sample edge denoted as the $x$ axis.

Figure 5

(a) Photocurrents $J_x(\alpha )/I$ measured in ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ sample for front and back illumination at $T=296\mathrm{K}$. Lines are fits after Eqs. (3) with $\mathrm{\Phi }=-4°$. Coefficient $A$ as a function of frequency $f$ (b) and the angle $\mathrm{\Theta }$ (c). Lines in (b) show fits after $A\propto 1/f^2$.